442 Dr. Johnston- Lavis — Mechanism of Volcanic Action. 



to separate them into dust, producing the pulverized material con- 

 stituting the essential part of the last and topmost deposit of an explosive 

 eruption. 



If the igneous paste rises from still greater depths it may come 

 from little or non- aquiferous parts of the conduit, which, together with 

 a higher specific heat from fewer losses thereof, will allow it to gush 

 forth in a non-fragmented state as a lava. 



The want of uniformity in the water-bearing rocks at different 

 depths is too well recognized for us not to see the influence on 

 possible departures and irregularities which might result in the 

 sequence of ejecta having the characters I have shown you as the 

 simplest expression of an eruptive phase. 



In an open chimney of a volcano in chronic activity the constant 

 circulation up the volcanic conduit allows of too little time for the 

 magma to acquire much volatile materials, and it is only when this 

 outflow is more or less impeded at the vent that more volatile materials 

 are acquired and the volcano assumes paroxysmal or explosive fits. 



In conclusion, I may say I have tried to summarize the trend of my 

 researches for the last thirty years, and if you will try to read the 

 whole phases of volcanic activity in this light you will find it the only 

 satisfactory explanation universally applicable to all cases of the eruptive 

 mechanism. No other theory that has been advanced has ever been 

 based on the characters of the actual essential ejecta, and no other one 

 fits without exception the whole range of the very varied phenomena 

 of volcanicity. 



EXPLANATION OF PLATES XXIV AND XXV. 



Pumices and pumiceous scoria from explosive eruptions of Monte Somma and 

 Vesuvius {essential ejecta). 



Plate XXIV. 

 Fig. 1. Light white pumice, bottom part of Phase III, period 1. The section is 

 seen to be mostly composed of_a clear glass with only an occasional 

 porphyritic crystal or microlith. Magnified 11 diameters. 



,, 2. Part of the same section, magnified 50 diameters. Most of the vesicles are 

 sectionized and open, and support the very rare porphyritic crystal by 

 sections of the thin shells of glass. A few vesicles still contain air. 



,, 3. Heavy chocolate-brown pumiceous scoria produced later by the same 

 eruption. Phase III. Notice how much smaller are the vesiculse and 

 how the mass of rock material is principally composed of microliths. 

 Magnified 11 diameters. 



,, 4. The same, magnified 50 diameters. Here the numerous large smooth- 

 walled vesicles of Fig. 2 are replaced by few small rough-walled spaces 

 into which the microliths project. 



Plate XXV. 



,, 5. Light white pumice, bottom part of Plinian pumice that buiied Pompeii, 

 Phase VII, period 1. The section shows the material to be mostly 

 a clear glass crowded by microliths of leucite only with few exceptions. 

 Magnified 11 diameters. 



,, 6. Part of same section, magnified 50 diameters, showing these characters 

 more accentuated. 



,, 7. Heavy greenish-grey pumice from high up in the pumice stratum. Here 

 the rock is composed of a dark, almost opaque network of microlithic 

 matter in which magnetite is abundantly distributed. The microbths 

 are almost hidden by the opacity of the magnetite and augite grains. 

 Magnified 11 diameters. 



